1. Field of the Invention
The present invention relates to magnetic detecting apparatuses, and more particularly, to a magnetic detecting apparatus for detecting by a magnetic method the positions of magnetic poles and a rotation velocity of a rotor of a motor and the like.
2. Background Art
Magnetic detecting apparatuses are conventionally known. An example of a conventional magnetic detecting apparatus is shown in the circuit diagram of FIG. 4. In the illustrated circuit, magnetic sensors 1-4 are provided, which consist respectively of a magneto-resistance element, i.e., a thin film device having an electrical resistance that varies in response to a magnetic field that is applied across it, also called a MR sensor. One terminal of each magnetic sensor 1 and 2, to which a power supply voltage V.sub.cc is applied, is connected together. The other terminal of the magnetic sensor 1 and 2 is connected to one terminal of each magnetic sensor 3 and 4, respectively. The other terminal of each magnetic sensor 3 and 4 is respectively grounded to the earth. The magnetic sensors 1-4 together form a circuit connected by a bridge. One input terminal of a differential amplifier 5 is connected to a terminal P.sub.a, namely, the connecting point to which the other terminal of the magnetic sensor 1 and one terminal of the magnetic sensor 4 is connected together. The other input terminal of the differential amplifier 5 is connected to a terminal P.sub.b, namely, the connecting point to which the other terminal of the magnetic sensor 2 and one terminal of the magnetic sensor 3 are connected to each other. The differential amplifier 5 detects the voltage between the terminals P.sub.a and P.sub.b, An output voltage of the differential amplifier 5 is outputted via an output terminal 6, such as in the case wherein, the detected voltage V.sub.0 indicates-the positions of the magnetic poles of a rotor of a motor whereat the magnetic sensors 1-4 are fixed, as shown in FIG. 5.
In the conventional magnetic detecting apparatus, even if the detected voltage V.sub.0 is not used as data, a current is always conducted through the magnetic sensors 1-4. Therefore, in the case where the detected voltage V.sub.0 is stored as backup data (i.e. background data), it is necessary to use only, as the magnetic sensors 1-4, a magneto-resistance element which uses a low amount of electricity and has a high resistance. Accordingly, in the case where normal magneto-resistance elements or Hall elements are used as the magnetic sensors 1-4, when using a battery, it is difficult to store data as the backup data.